The present invention relates in general to control valves, and, more specifically, to a double valve for controlling a single flow of pressurized fluid in response to simultaneous activation of a pair of control switches.
Machine tools of various types operate through a valving system, which interacts with a pneumatically-controlled clutch and/or brake assembly. For safety reasons, the control valves that are used to operate these machine tools require the operator to activate two separate control switches substantially simultaneously to ensure that an operator's hands are away from the moving components of the machine tool when an operating cycle is initiated. Typically, an electronic circuit responsive to the two control switches generates a pilot control signal applied to the pilot valves for switching the main fluid circuit of the valve to control delivery of compressed air (or other fluid) to the machine tool to perform its operating cycle.
Double valves operating in parallel in one valve body have been developed to ensure that a repeat or overrun of a machine tool operating cycle cannot be caused by malfunction of a single valve unit (e.g., a valve becoming stuck in an actuated position). Thus, if one valve unit fails to deactuate at the proper time, the double valve assumes a configuration that diverts the source of compressed air from the machine tool. A double valve is shown, for example, in commonly assigned U.S. Pat. No. 6,478,049 to Bento et al, which is incorporated herein by reference for all purposes.
In addition to providing protection against the repeat or overrun of the machine tool, it is desirable to monitor the double valve for a faulted valve unit and to prevent a new operating cycle of the machine tool from being initiated. Thus, prior art systems have caused the double valve to assume a lock-out configuration when a single valve unit is in a faulted condition so that the double valve cannot again be actuated until it has been intentionally reset to clear the faulted condition.
More specifically, a double valve assembly includes two electromagnetically-controlled pilot valves. Typically, the pilot valves are normally closed. The double valve assembly includes two movable valve units, each with a respective exhaust poppet between the outlet port and the exhaust port of the double valve and a respective inlet poppet between the outlet port and the inlet port of the double valve. When the pilot valves are normally closed, then the exhaust poppets are normally open and the inlet poppets are normally closed. Each of the pilot valves is moved to an actuated position in response to an electrical control signal from a respective operator-controlled switch, which typically causes the exhaust poppets to close and the inlet poppets to open. Any time that 1) a valve unit fails to deactuate properly, 2) a valve unit fails to actuate properly, or 3) the pilot valves are actuated or deactuated non-simultaneously, then at least one valve unit becomes locked in a faulted position where its exhaust poppet cannot be closed (thereby preventing the outlet from becoming pressurized).
During normal running conditions, the inlet to the double valve receives a continuous source of pressurized fluid. However, the source is periodically turned off (e.g., during maintenance or at the end of a work shift). When the pressurized fluid cycles off and on, pressures within different sections of the double valve acting upon various valve components decays and then rebuilds, thereby causing forces on the valve units not typically experienced during normal running conditions. In prior art double valves, the affect upon the movable valve units of cycling the pressure has typically been inconsistent and unpredictable. In many instances, a valve unit that was in a faulted state can end up being reset by the pressure cycling. This is undesirable because the failure of a valve that becomes faulted shortly before cycling the pressure might not be noticed before the pressure is turned off. If the faulted valve is reset by the pressure cycling, then the indication of a malfunction is lost and it may be possible for a valve that should be locked out to attempt to operate normally. On the other hand, it is also possible for a non-malfunctioning valve unit to inadvertently assume the faulted position when no fault has actually occurred, thereby requiring valves to be reset after cycling the pressure off and on which adds inefficiency in a manufacturing operation. Consequently, it would be desirable to provide a dynamic memory of the valve state during the cycling of inlet pressure so that each valve unit resumes the same state as it had when the pressure was removed.